A static pressure lead screw is known as a means for achieving high-speed, high-accuracy feeding for converting rotational movement into linear movement. In the static pressure lead screw, a pressurized fluid is supplied between male and female screws or external and internal threads to support them through the pressurized fluid. A fluid bearing is also proposed to supply a pressurized fluid to a gap between a rotating shaft and a bearing for supporting it. These mechanical elements are so-called static pressure guides. The present invention particularly relates to a porous static pressure guide comprising a porous body for a bearing surface, a static pressure lead screw, and a feed mechanism using the same.
Conventional static pressure lead screws as one type of static pressure guides will be briefly described below. A static pressure lead screw serves as a static pressure guide wherein a pressurized fluid is supplied to a gap between male and female screws (this gap is referred to as a screw gap hereinafter) to support the male and female screws through the pressurized fluid in a noncontact manner. The static pressure lead screws are classified into a hydrostatic lead screw and a pneumostatic lead screw according to the types of working fluid. These static pressure lead screws are not subjected to solid friction and are free from stick-slip and backlash. Feed accuracy can be greatly improved by a fluid film averaging effect. In addition, since no solid friction occurs in the static pressure lead screw, accuracy is not permanently degraded. For these and other reasons, in recent years, the static pressure lead screws have received a great deal of attention for a variety of applications in high-accuracy, high-speed apparatuses such as apparatuses for manufacturing semiconductor elements and optical components and electronic equipment.
However, since the fluid is supplied, the structure of the female screw is complicated. In addition, design is difficult and more accurate machining is required, thus posing many problems to be solved. Therefore, practical static pressure lead screws have not yet been proposed. In particular, it is difficult to increase stiffness of an air lubrication film between the male and female screws in a pneumostatic lead screw using a gas of a very low viscosity. No practical pneumostatic lead screws are proposed.
The structures of conventional hydrostatic lead screws are described in Journal of the Japan Society of Precision Engineering (JSPE) 49, July (1983), P. 889 and Journal of the Japan Society of Precision Engineering 48, October (1982), P. 1291.
Oil supply holes directly influence an amount of fluid supplied to a screw gap and uniformity of the flow and directly dominate the performance of the hydrostatic lead screw. High machining accuracy of the oil supply holes is required. When the hydrostatic lead screw is to be made compact, it is difficult to precisely form helical oil supply and recovery holes in small mechanical parts. Therefore, compactness of the hydrostatic lead screws cannot be easily achieved.
In the pneumostatic lead screw using a gas (e.g., air) as a working fluid, statical stiffness between the male and female screws is more important than in the hydrostatic lead screw. This is because the viscosity coefficient of the gas is as extremely small as the order of 1/1,000 and the used gas is a compressible fluid. For these reasons, the screw gap in the pneumostatic bearing must be smaller than that in the hydrostatic bearing and must fall within the range of several microns to a value between 10 and 20 microns. The pneumostatic lead screw uses a flank as an air bearing surface. In order to obtain the gap described above, surface flatness, geometrical accuracy, and pitch accuracy must be very high in machining of the female and male screws. In addition, orifice restrictors must be formed for a large number of air supply holes to give statical stiffness to the lead screw. Difficulties in complicated more accurate machining result in a bulky pneumostatic lead screw at high cost. A practical pneumostatic lead screw cannot be realized.
FIGS. 13(a) and 13(b) are sectional views of conventional pneumostatic lead screws (Japanese Patent Laid-Open (Kokai) Nos. 59-113360 and 60-241566). Exhaust paths are not illustrated in FIGS. 13(a) and 13(b).
Referring to FIGS. 13(a) and 13(b), nuts 3 are made of porous bodies and serve as porous restrictors, respectively. The pneumostatic lead screws in FIGS. 13(a) and 13(b) have similar structures. However, in the pneumostatic lead screw in FIG. 13(b), portions (indicated by thick lines) excluding the flanks are sealed. With this structure, air supply holes need not be formed, as shown in the lead screw in FIG. 13(a). Otherwise, the air supply holes in FIG. 13(b) are formed by simply boring the porous body. In addition, the flanks are formed by simple thread cutting. Since the porous restrictors have high air supply efficiency and the statical stiffness of the pneumostatic lead screw can be improved as compared with lead screws employing other types of restrictor. However, when the porous body is a metal porous body, clogging during drilling or cutting may pose a problem due to plastic deformation of the material. When the flanks 9 in FIGS. 13(a) and 13(b) or the wall surfaces defining air supply holes 6 in FIG. 13(b) clog, the air flow paths are blocked, and the lead screws do not serve as pneumostatic lead screws. Proper clogging of an air blow surface in a porous air bearing is required to improve the axial stiffness as in the orifice restrictor. If a free margin is given in machining of the bearing surface in a planar bearing such as a linear thrust bearing, such a clogging effect can be effectively utilized. However, when the floating screw gap is determined by the machining amount due to the dimensional limitations of opposite bearing surfaces as in a radial bearing, a pneumostatic lead screw, or the like, it is difficult to control proper clogging during machining. Furthermore, a workpiece clogging amount varies depending on various conditions such as the types of material, the cutting depth, and a machining speed. Therefore, both the clogging amount and machining accuracy cannot be simultaneously satisfied.
The externally pressurized air bearing has a problem of pneumatic hammer. "Pneumatic hammer" is a self-excited vibration caused by compressibility of the gas. The pneumatic hammer tends to occur in a gas bearing having a large flow path volume, such as a pocketed orifice restrictor and a porous restrictor. The nuts in FIGS. 13(a) and 13(b) are made of porous bodies and have large flow path volumes. Therefore, the pneumatic hammer tends to occur in these lead screws.
Pore collapsing in a conventional porous bearing will be briefly described. In a conventional externally pressurized porous bearing, a sectional area of a porous surface layer for causing a fluid to pass therethrough is reduced by pore collapsing, as shown in FIG. 14. The porous body is prepared by sintering particles of a metal powder at a temperature lower than a melting point, and spaces defined by the bonded particles constitute a fluid path. The porous body is easily accessible at low cost since it has been used in a variety of applications in sintered oil-containing bearings and filters.
Pore collapsing in the porous metal body is performed by utilizing plastic deformation. More specifically, cutting or grinding is performed by selecting cutting conditions such as sharpness of a cutting edge of a cutting tool, a cutting depth, a machining speed, or a machining temperature. The sectional area of the fluid path present on the porous body surface can be decreased by plastic deformation. Therefore, an amount of plastic deformation corresponds to an increase in flow resistance. Since numerous large and small holes are present on the the porous body surface, the small pores are collapsed when an amount of plastic deformation is increased. Semi-closed pores and almost open pores are left according to a given probability. This is the principle of surface layer restrictor by pore collapsing. The flow resistance of the porous body surface layer is controlled by an amount of plastic deformation.
Pore collapsing by plastic deformation is limited to a material subjected to plastic deformation, and the machining conditions must be properly selected. Variations in sintering strength and hardness adversely affect machining conditions. Friction and wear of the machining tools fail to guarantee good machining conditions. Reproducibility and productivity are poor since plastic deformation itself is based on machining with skills or machining according to a given probability. In this sense, a uniform pore-collapsed surface cannot be formed. In particular, when an amount of plastic deformation is near the particle size of the powder constituting the porous body, removal and reattachment of particles simultaneously progress. The wear of the cutting tool and damage thereto are increased. It cannot be expected to accurately control pore collapsing accuracy of the pore-collapsed surface. Great degradation of machining accuracy of the pore-collapsed surface is inevitable. Plastic deformation is a factor opposite to improvement of machining accuracy. However, as described above, machining accuracy improves performance of the static pressure guide itself directly. Pore collapsing by plastic deformation is a surface layer restrictor formation technique controversial to high performance of the static pressure guide.
Pore collapsing greatly changes the flow resistance of the porous body. In order to assure statical stiffness o the static pressure guide, a flow path resistance ratio must be set such that a peak value of an intermediate pressure Pm is given to be about 2/3 the supply pressure Ps. Control must cover control of the flow resistance of the overall porous restrictor in addition to control of uniform pore collapsing of the porous body surface. For this reason, it is not going to far to say that accuracy of pore collapsing determines the performance of the static pressure guide. However, practical static pressure guides rarely employ porous restrictors. Conventional surface restrictors are most popular in static pressure guides. This fact explicitly indicates that problems for controlling the flow resistance to achieve porous restrictors and for forming surface layer restrictors are left unsolved.